This invention pertains to a method of heat-treating poly(methyl methacrylate) or PMMA structures fabricated by radiation lithography, such as X-ray, electron-beam, or ion beam, to reduce the surface roughness of the bottom and side walls.
The primary use of radiation-exposed PMMA structures is in the LIGA process to produce microstructures. LIGA is a German acronym for lithography, electroforming, and molding. In the first step, a photoresist polymer layer from several microns to a few thousand microns thick, either on a substrate or freestanding, is irradiated through a mask, usually by X-ray, electron-beam, or ion beam radiation. Irradiation changes the characteristics of the polymer (e.g., increases its solubility), such that the irradiated polymer can be removed when developed in an appropriate solution while the unexposed resist remains. After developing, a relief-like polymer structure is left, usually attached to a electroconductive substrate (or it may be glued to a substrate if it was irradiated as a freestanding structure).
In the next step, metal is electro-deposited on the substrate in the spaces in between the remaining polymer structure, giving a metal structure complimentary to the polymer relief and mirroring the original mask shape. The resulting a microstructural element produced may then be used as a mold for casting in mass production, usually in a polymer. One difficulty with the LIGA process as originally described was that it only produced microstructures that were attached to a substrate.
To produce microstructures with a high aspect ratio, deep X-ray lithography is used. Deep X-ray lithography involves a substrate covered by a thick photoresist, typically several thousand microns in thickness, which is exposed by X-rays through a mask. The high energy of X-ray photons makes exposure of thick photoresist films with high throughput feasible. In direct, high throughput LIGA processes, many sheets of PMMA may be stacked together to utilize the deep X-ray beam energy in mass producing exposed PMMA resists. After the exposure, the individual PMMA sheets are solvent bonded to working substrates before further processing. See H. Guckel et al., xe2x80x9cDirect, high throughput LIGA for commercial applications: A progress report,xe2x80x9d Book of Abstract, HARMST ""99, Kisarazu, Japan, Jun. 13-15, 1999, p. 2-3 (1999). Deep X-ray lithography allows structures to be produced with heights up to several mm and a lateral resolution down to 0.2 xcexcm or smaller.
Poly(methyl methacrylate) (PMMA) is the most commonly used photoresist material in X-ray, deep X-ray, and electron-beam lithographies. See Y. Vladimirsky et al., xe2x80x9cPMMA as an X-ray resist for micro-machining application: Latent image formation and thickness losses,xe2x80x9d Microelectronic Engineering, vol 30, pp. 543-546 (1996).
In addition, because of its superior resolution, contrast, mechanical behavior, and optical properties, PMMA is widely used as a structural material for micro-analytical instruments, micro-actuators for medical applications, micro-machine parts, optical components, in prototyping applications, and low-to-medium volume manufacturing. See L. E. Ocola et al., xe2x80x9cParametric modeling of photoelectron effects in X-ray lithography,xe2x80x9d Journal of Vacuum Science and Technology B, vol. 11, pp. 2839-2844 (1993); U. Gebhard et al., xe2x80x9cCombination of a fluidic micro-oscillator and micro-actuator in LIGA-technique for medical application,xe2x80x9d TRANSDUCERS ""97, Chicago, Jun. 16-19, 1997, pp. 761-764 (1997); and C. Muller et al., xe2x80x9cMicrospectrometer fabricated by the LIGA process,xe2x80x9d Interdisciplinary Science Reviews, vol. 18, pp. 273-279 (1993).
A drawback to the use of PMMA has been that the bottom and side-wall surfaces of PMMA structures prepared by X-ray exposure and then developed are rough and contain crack-like fissures. See C. K. Malek et al., xe2x80x9cMetrology study of structural transfer accuracy in fabrication of high-aspect-ratio microelectromechanical systems: From optical mask to polished electroplated parts,xe2x80x9d J. Vac. Sci. Technol. B, vol. 16(6), pp. 3552-3557 (1998); and C. M. Egert et al., xe2x80x9cDimensional variation and roughness of LIGA fabricated microstructures,xe2x80x9d SPIE, vol. 2880, pp. 200-209 (1996). Another study had shown micropore formation on PMMA after an X-ray exposure. See A. L. Bogdanov et al., Mikroelektronika, vol. 17, p. 261 (1988). These rough surfaces directly affect the performance (e.g., optical properties), signal-to-noise ratio, and even the availability of these devices and components. Additionally, the rough surface makes it difficult to glue the exposed PMMA components onto another surface thus limiting the ability to assemble more complex microstructures as envisioned in U.S. Pat. No. 5,496,668. The formation of surface roughness and fissures make batch fabrication of multi-layer PMMA devices and components almost impossible, because of the inability to glue the exposed PMMA structures to a suitable substrate. See C. G. K. Malek et al., xe2x80x9cFactors promoting the adhesion of acrylic sheets to metallic structures for LIGA processing,xe2x80x9d Book of Abstract, HARMST ""99, Kisarazu, Japan, Jun. 13-15, 1999, p. 110-111 (1999). See also, M. Chabloz et al., xe2x80x9cImprovement of sidewalls roughness in deep silicon etching,xe2x80x9d Book of Abstract, HARMST ""99, Kisarazu, Japan, Jun. 13-15, 1999, p. 26-27 (1999); S. K. Griffiths et al., xe2x80x9cThe influence of mask substrate thickness on exposure and development times for the LIGA process,xe2x80x9d Book of Abstract, HARMST ""99, Kisarazu, Japan, Jun. 13-15, 1999, p. 86-87 (1999); F. J. Pantenburg et al., xe2x80x9cImproved adhesion of deep X-ray lithography resist structures by using a mirror system,xe2x80x9d Book of Abstract, HARMST ""99, Kisarazu, Japan, Jun. 13-15, 1999, p.122 (1999); C. K. Malek et al., xe2x80x9cLithographic related problems in deep X-ray lithography: Radiation-induced formation of volatile species in poly(methylmethacrylate) (PMMA),xe2x80x9d Book of Abstract, HARMST ""99, Kisarazu, Japan, June 13-15, 1999, p. 112-113 (1999). To fully utilize the potential of PMMA as both a resist and structure material in LIGA, a method to modify and decrease the surface roughness of PMMA structures after radiation exposure is needed. U.S. Pat. No. 4,393,129 describes a method to develop (using the GG developer) exposed resist layers, particularly composed of PMMA, to avoid stress cracks, especially in structures with a high aspect ratio produced by X-ray or electron-beam radiation.
We have discovered a method to decrease the surface roughness of radiation-exposed PMMA surfaces by following the exposure step with a heat treatment. The PMMA surface roughness was decreased by post-exposure heat treatments less than 70xc2x0 C. The optimum post-exposure heat treatment to produce a PMMA microstructure with a smooth surface was found to be about 60xc2x0 C. for 30 min. Aside from the enhanced smoothness, the structural features of post-exposure heat-treated PMMA patterns were shown not to be statistically different from identical features of untreated post-exposure PMMA patterns. The post-exposure heat treatment produces a smoother PMMA structure that can then be glued onto a substrate, or that can be assembled with other PMMA structures into a three-dimensional or multilayer microstructure.